Bone density (or bone mineral density) is a medical term normally referring to the amount of mineral matter per square centimeter of bones.[1] Bone density (or BMD) is used in clinical medicine as an indirect indicator of osteoporosis and fracture risk.
This medical bone density is not the true physical "density" of the bone, which would be computed as mass per volume. It is measured by a procedure called densitometry, often performed in the radiology or nuclear medicine departments of hospitals or clinics. The measurement is painless and non-invasive and involves low radiation exposure. Measurements are most commonly made over the lumbar spine and over the upper part of the hip.[2] The forearm may be scanned if the hip and lumbar spine are not accessible. Average density is around 1500 kg m−3
There is a statistical association between poor bone density and higher probability of fracture. Fractures of the legs and pelvis due to falls are a significant public health problem, especially in elderly women, leading to much medical cost, inability to live independently, and even risk of death. Bone density measurements are used to screen women for osteoporosis risk and to identify those who might benefit from measures to improve bone strength.
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Results are often reported in 3 terms:
Use of BMD has several limitations.
The National Osteoporosis Foundation recommends BMD testing for the following individuals:[3]
While there are many different types of BMD tests, all are non-invasive. Most tests differ in which bones are measured to determine the BMD result.
These tests include:
DEXA is currently the most widely used, but ultrasound has been described as a more cost-effective approach to measure bone density.[4]
The test works by measuring a specific bone or bones, usually the spine, hip, and wrist. The density of these bones is then compared with an average index based on age, sex, and size. The resulting comparison is used to determine risk for fractures and the stage of osteoporosis in an individual.
Average bone mineral density = BMC / W [g/cm2]
Results are generally scored by two measures, the T-score and the Z-score. Scores indicate the amount one's bone mineral density varies from the mean. Negative scores indicate lower bone density, and positive scores indicate higher.
The T-score is a comparison of a patient's BMD to that of a healthy thirty-year-old of the same sex and ethnicity. This value is used in post-menopausal women and men over aged 50 because it better predicts risk of future fracture.[5] The criteria of the World Health Organization are:[6]
| WHO category | Age 50-64 | Age > 64 | Overall |
|---|---|---|---|
| Normal | 5.3 | 9.4 | 6.6 |
| Osteopenia | 11.4 | 19.6 | 15.7 |
| Osteoporosis | 22.4 | 46.6 | 40.6 |
The Z-score is the number of standard deviations a patient's BMD differs from the average BMD of their age, sex, and ethnicity. This value is used in premenopausal women, men under the age of 50, and in children.[5]
There may be associations between some forms of BMD and diet[8]
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